Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras have required two connections: one to a LAN and another to a power supply system. A PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
The PoE system must comply with the IEEE 802.3af standard that defines delivering power over unshielded twisted-pair Ethernet wiring from a PSE to a PD located at opposite sides of a link. As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
The main functions of the PSE are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting a PoE detection signature defined by the IEEE 802.3af standard.
If the detection signature is valid, the PD has an option of presenting a classification signature to the PSE to indicate how much power it will draw when powered up. A PD may be classified as class 0 to class 4. A PD of class 1 requires that the PSE supplies at least 4.0 W, a PD of class 2 requires that the PSE supplies at least 7.0 W, and a PD of class 0, 3 or 4 requires at least 15.4 W. Based on the determined class of the PD, the PSE applies the required power to the PD.
A pass device, such as a MOSFET, may act as a switch between the PSE and the PD. During power-up and short-circuit conditions, power dissipation in the MOSFET may be much higher than power dissipation when nominal power is provided. To limit the power dissipation, a foldback mechanism is prescribed by the IEEE 802.3af standard. In particular, the standard defines that in a startup mode, for port voltages between 10V and 30V, the minimum requirement for an output current (IInrush) is 60 mA. For port voltages above 30 V, the current IInrush in a startup mode is required to be in the range from 400 mA to 450 mA. This 400 mA to 450 mA IInrush requirement applies for duration of the 50 ms to 75 ms TLIM timer.
FIG. 1 shows a diagram that graphically illustrates the IEEE 802.3af foldback requirements. In particular, the gray areas in FIG. 1 show combinations of PSE output voltages and output currents that are not allowed by the IEEE 802.3af standard. The black line in FIG. 1 illustrates a possible foldback curve representing the output current of the PSE at a level between 400 mA and 450 mA for output voltages above 30V and gradually reduced for output voltages below 30V. The current limit foldback technique is used to limit dissipation power, and therefore, size and cost of the pass device.
However, for a high-power PSE capable of providing higher power to the PD than power mandated by the IEEE 802.3af specification, current limits should be higher than the 400 mA to 450 mA current prescribed by the IEEE 802.3af standard. Because higher currents cause higher MOSFET power dissipation, a larger pass device would be required. It would result in higher costs for the PSE manufacturer.
Therefore, there is a need for a foldback mechanism that would reduce power dissipation of a pass device in a high-power PSE.